Couple of things:
1. Lot was not Abraham's brother. He was Abraham's nephew:
"27 This is the genealogy of Terah: Terah begot Abram, Nahor, and Haran. Haran begot Lot. 28 And Haran died before his father Terah in his native land, in Ur of the Chaldeans. 29 Then Abram and Nahor took wives: the name of Abram’s wife was Sarai, and the name of Nahor’s wife, Milcah, the daughter of Haran the father of Milcah and the father of Iscah. 30 But Sarai was barren; she had no child.
31 And Terah took his son Abram and his grandson Lot, the son of Haran, and his daughter-in-law Sarai, his son Abram’s wife, and they went out with them from Ur of the Chaldeans to go to the land of Canaan; and they came to Haran and dwelt there. 32 So the days of Terah were two hundred and five years, and Terah died in Haran."
Genesis 11
2. Lot WAS a righteous man. He lived in Sodom, but there is absolutely no Biblical evidence that he participated in the evil that went on in that place. In fact, the Bible says that Lott:
"who was oppressed by the filthy conduct of the wicked 8 (for that righteous man, dwelling among them, tormented his righteous soul from day to day by seeing and hearing their lawless deeds)— "
2nd Peter 2
Lot never participated in the lawless deeds, he merely saw them, and he was TORMENTED by them.
As to whether or not God intended to destroy Sodom with Lot in it, I don't think one can make such a case Biblically. God spoke with Abraham because He knew Abraham would intercede on Lot's behalf. Remember that Abraham prayed for God to SPARE the city for the sake of the righteous. He did NOT pray that God would take Lot out of the city. God was revealing something about Himself to Abraham just as He would later do when He commanded Abraham to sacrifice Isaac.
3. Was Lot corrupt? Not really. The Bible does not call people righteous if they are corrupt. Lot did not share the moral failings of the people of Sodom. This is not to say that Lot was perfect, as no person the Bible calls righteous is perfect. (Save Christ Himself) Lot chose to settle on the plains of Sodom because he thought the trade would be beneficial to him. He knew the place was wicked, but most likely believed that he could remain pure in the wicked environment.
Lot's offer of his daughters is not a sign of corruption. It may seem that way to some, but remember that Lot was also trying to protect strangers who happened to be messengers of The Lord. What Lot was doing was actually a courageous and moral thing according to the culture of his day. It seems unthinkable to us, but things were different 4,000 years ago. In taking these strangers in and offering his daughters, he was trying to shield them from the dangers and moral horrors of Sodom, which was a righteous thing.
Lot's impregnation of his daughters is also not a sign of corruption. Remember that Noah got drunk and passed out naked after discovering wine, but God did not call Noah corrupt. Lot's daughters, like their mother, were corrupt.
The lesson of Lot is that if even a righteous man desires the benefits of the worldly system that it can corrupt everything he has and in the end he will have to choose between it and God. It also shows that there is a difference between being righteous and truly knowing God, as Lot was righteous but unaware that he was in the path of Divine Judgement while Abraham knew what God was going to do and even had a hand in negotiating it with God.